"automatically resize to fit contents" grayed out

I have a nested table within which I need to include a URL
for reference, however I do NOT want the width of the cell
to resize to match the width of the (considerably long)
URL. I know I should uncheck the "automatically resize to
fit contents" checkbox in the table options dialog,
however, it's grayed out, and checked, in the dialog and I
can't figure out how to uncheck it.

Oh, and it is unchecked for the table that holds this
nested table. Any tips??!!

Thanks,
T